A known drum-type centrifuge filter has a sieve drum rotatable about an axis and an inner sieve surface normally formed by a filter medium, the interior of the drum constituting an axially forwardly open sieve chamber. A filtrand is fed into the sieve chamber while the drum is rotated about its axis so as to centrifugally separate the filtrand into solids lying on the sieve surface and a liquid phase that is radially discharged through a perforated peripheral wall of the drum.
In order to clear the sieve chamber of solids retained by the filter medium inside the drum, a pusher plate is provided which is axially displaceable in the drum between a rear end position and a front end position. This pusher plate has an outer edge closely juxtaposed with the inner surface of the peripheral drum wall so that the plate, on moving axially from its rear position to its front position, scrapes the solids from the filter medium and thrusts them forward out of the chamber. Normally the pusher plate is operated by a piston mounted in a cylinder that is rotationally fixed to the drum and forms a cylinder chamber which is centered on its axis but is axially offset from the sieve chamber. The piston axially subdivides this cylinder chamber into a pair of compartments.
The filter-cake solids are scraped from the inner sieve surface by pressurizing the rear compartment between the piston and a wall of the cylinder so as to move the pusher plate forward. The pusher plate is then returned to its rest or rear position by pressurizing the front compartment. This operation normally takes place while the filtrand is continuously fed into the device which continues to rotate.
Oil is fed to the two chambers through a bearing that axially fits on the shaft of the drum and communicates through axial passages in the shaft with respective radial passages opening into the cylinder compartments. A simple reversing valve fills one compartment while emptying the other for the above-described axial reciprocation of the piston with the pusher plate.
Since a substantial axial force must be exerted by the pusher plate in order to scrape the solids from the inner drum surface, the oil must be introduced under considerable pressure, so that high-quality seals must be provided for the piston and the various connections. Moreover, it is normally necessary to provide a cooling device for the oil which becomes hot during use. Thus, such an arrangement is extremely expensing and, owing to its complexity, failure-prone.
Various drum centrifuges of the above-described general type are described in U.S. Pat. Nos. 1,382,142 and 3,415,446, as well as in French Pat. Nos. 887,232 and 963,142 respectively dated Oct. 22, 1942 and Mar. 22, 1948. Others are described in Swiss Pat. No. 220,555 of Apr. 15, 1942 and in East German Pat. No. 27,446 of Mar. 5, 1964; see also West German Pat. Nos. 625,067, 648,087, 2,220,718, 2,508,017, 2,521,838 and 2,538,630.